Process Equipment and Plant Design
In the case of a shell and tube heat exchanger, the logarithmic mean temperature difference

Is always less than arithmetic average value
Is always less than arithmetic mean value, but more than geometric mean value
Is always more than arithmetic mean value and the geometric mean value
May be either more or less than geometric mean and arithmetic mean value depending upon whether the flow of stream is co-current or counter-current

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Process Equipment and Plant Design
Ponchon Savarit method is based on the use of enthalpy concentration diagram, which contains the bubble point curve (saturated liquid curve), dew point curve (saturated vapour curve) and equilibrium tie lines. As compared to McCabe-Thiele's method, this method

Accounts for the enthalpy changes in the process
All of these
Facilitates direct calculation of heat load on reboiler & condenser from, the diagram used in this method
Is more accurate in finding the number of equilibrium stages

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